The design of an active pixel sensor test structure optimised for the read out of scintillator screens

Indirect detection of X-rays using a scintillator coupled to a Solid-state sensor is an application where the CMOS Active Pixel Sensor (APS) can offer improved performance compared with conventional charge coupled device (CCD) based systems. The main advantages that CMOS sensors offer are reduced susceptibility to radiation damage, lower power consumption and increased on chip functionality. We describe the operation of a basic photodiode type pixel and explain how the read-out and sensing arrangement can be optimised for indirect X-ray detection in digital radiography applications. The optimised pixel is designed to have a narrow-band response to visible light generated by a scintillator screen, while at the same time suppressing signal from direct X-ray interaction. We outline a programme that has recently begun at Brunel University and e2v technologies to design, fabricate, and characterise variants of these pixel structures to perform a parametric study of CMOS imager performance.